In an aircraft or a coach where passengers are always involved with noises, the passengers at the seats sometimes cannot clearly catch information provided through audio, such as an in-flight notice, due to the noises around the seats.
The aircraft or the coach defines an interior space with continuous walls, so that the interior space forms a kind of hermetic structure. If noise sources exist inside and outside the interior space, the passengers in the interior space are to be confined within a regular noise environment. An excess noise sometimes invites physical or mental stress to the passengers, thereby degrading the comfortableness in the interior space. In the case of an aircraft, in particular, although flight attendants try to provide the passengers with good service in the interior space, the noise becomes a critical problem to a service quality.
In the case of the aircraft, the following noises are chiefly involved: noises produced by the devices such as a propeller or an engine which generates thrust force for the aircraft, and noises, such as zip sound, involved with airstream produced by the movement of the aircraft in the air. The foregoing noises audible in the interior space make the passengers unpleasant and also hinder the in-flight audio notice. The noises thus need to be reduced.
Passive attenuating measures have been taken, in general, for reducing the noises in the hermetic space. This method places sound insulating material, such as a diaphragm or sound absorption material, between the hermetic structure and the noise source. The diaphragm includes, e.g. a high density diaphragm, and the sound absorption material includes, e.g. an acoustical sheet, which is, however, a high density member and thus becomes a weight gaining coefficient. An increment in the weight consumes a greater amount of fuel or reduces a flight range. As a result, the increment in the weight incurs degrading the economical performance of the aircraft. On top of that, the foregoing materials have a problem of strength such as being subject to damages and a problem of design such as having a poor quality image.
To overcome the disadvantages of the foregoing passive attenuating measures, a noise control device has been recently proposed. This noise control device reproduces a control sound having a reverse phase to that of a noise arriving at a control point, thereby reducing the noise (an active noise control disclosed in e.g. Patent Literature 1). This control method is achieved by operating a fixed filter and an adaptive filter selectively.
The conventional noise control device discussed above is detailed hereinafter with reference to FIG. 15, which shows a circuit diagram of the conventional noise control device. In FIG. 15, the noise control device includes noise microphone 9101, adaptive filter 9201, control speaker 9401, error microphone 9501, and fixed filter 9601.
The noise control device shown in FIG. 15 selects adaptive filter 9201 for performing a noise control when noises are varied due to a position change of a noise source or a change in a noise production state, e.g. a change in a driving condition or an rpm of a fan. Noise microphone 9101 detects coming-noises supplied from a noise source, and then outputs a noise signal to adaptive filter 9201. Filter 9201 processes the noise signal by using a filter coefficient, thereby generating a control signal, which is then radiated as a control sound from speaker 9401 to a control point. Error microphone 9501 is placed at the control point for detecting the noise supplied from the noise source and arriving at the control point as well as the control sound supplied from control speaker 9401 and arriving at the control point. At error microphone 9501, the noise arriving at the control point interferes with the control sound supplied from control speaker 9401, and the difference between these noise and sound is detected as an error signal. Adaptive filter 9201 renews its own coefficient such that the error signal can be minimized. The renewal is done, e.g. by a Filtered-X_LMS method, which is referred to as a coefficient renewal process hereinafter. Adaptive filter 9201 thus can renew its own filter coefficient such that an optimum control signal can be generated in response to the noise having undergone the following change and arriving at the control point when the noise is changed due to the position change of the noise source or a change in the noise producing condition.
When the renewed filter coefficients converge on one coefficient, the noise control device shown in FIG. 15 selects fixed filter 9601, at which the converged filter coefficient is fixedly set, thereby controlling the noise. The noise control device shown in FIG. 15 thus operates the fixed filter or the adaptive filter selectively for carrying out the active noise control.
For instance, the noise typically representing the engine noise in an aircraft has an almost constant noise level, so that the filter coefficient scarcely needs to be renewed. However, if a passenger beats around the error microphone so that a noise of different level can occur momentarily, then adaptive filter 9201 renews the coefficient such that the noise can be cancelled instantaneously. This mechanism thus allows the control sound to adversely affect, so that it is afraid that the noise level can be higher than a noise level where the control sound is not yet reproduced, i.e. a level before the noise is controlled.
The noise control device shown in FIG. 15 selects adaptive filter 9201 for controlling the noise, so that it needs a circuit which can perform the coefficient renewal. As a result, the circuit cannot be downsized. On top of that, the renewal of the coefficient of adaptive filter 9201 needs to calculate the coefficient on a real time basis, so that a strict processing capability is required. What is worse, if a wrong filter coefficient is used, a wrong control sound is reproduced immediately, and the noise level becomes higher than that when the control sound is not reproduced, i.e. before the noise is controlled, and resultantly makes the passengers sometimes unpleasant.